The present invention relates to an ultrasonic horn used in wire bonding apparatuses or flip-chip bonding apparatuses or the like.
As shown in 24(a), an ultrasonic horn 10 used in a wire bonding apparatus or flip-chip bonding apparatus or the like is generally attached with a capillary 12, flanges 13, and ultrasonic vibrator 11.
The ultrasonic horn 10, which has a taper formed therein for the purpose of amplitude magnification, constitutes a truncated cone, so that it becomes gradually narrower toward the front end. The ultrasonic vibrator 11 is secured to the rear end portion of the ultrasonic horn 10. Ultrasonic vibration generated by this ultrasonic vibrator 11 is conveyed along the ultrasonic horn 10 as a longitudinal wave, and the ultrasonic horn 10 resonates at a resonant frequency determined by the shape and material thereof, so that a standing wave is generated. In the ultrasonic horn 10, a large-amplitude portion 16 and a node 18, where no amplitude occurs, are generated. FIG. 24(b) illustrates the amplitude generated in an ultrasonic horn, in which the amplitude of a plane (in the fore and aft dimension) perpendicular to the horn centerline of the ultrasonic horn is indicated on the vertical axis. The capillary 12 used for bonding is attached to the position 16, at the front end portion, where the amplitude becomes large. The ultrasonic horn 10 is attached to a wire bonding apparatus or a flip-chip bonding apparatus or the like by the flanges 13 attached at the position of the node 18 where no amplitude occurs.
FIG. 29(a) shows an ultrasonic horn 60 used in flip-chip bonding. In this ultrasonic horn 60, a tool (bonding tool) 61 that is a processing device is attached in the middle portion of the ultrasonic horn 60, and the ultrasonic vibrator 11 is attached to the rear end portion 65 of the ultrasonic horn 60. The ultrasonic horn 60, for the purpose of amplitude magnification, is designed so that it becomes narrower from both ends toward the middle portion where the tool 61 is attached.
The ultrasonic vibration generated by the ultrasonic vibrator 11 is conveyed along the ultrasonic horn 60 as a longitudinal wave, is reflected by the front end portion 64, and generates a standing wave in the ultrasonic horn 60. FIG. 29(b) is a diagram representing the manner of this standing wave. At the rear end portion 65 where the ultrasonic vibrator 11 is attached, and at the front end portion 64 where the ultrasonic wave is reflected, antinodes 63 where the amplitude of the standing wave is large are formed; and the portion where the tool 61 is attached also constitutes an antinode 63 and vibrates fore and aft. On the two sides of the tool 61, meanwhile, nodes 18 are generated where no amplitude occurs. At the positions of these nodes 18, the flanges 13 are formed. The flanges 13 have U-shaped attaching portions 70 on the outside thereof, so that the ultrasonic horn 60 is attached to a flip-chip bonding apparatus via the attaching portions 70.
As seen from FIG. 25, when ultrasonic horns 10 and 60 as described above are in a resonant state, a resonance-induced compressive stress P1 from the rear end portion and a resonance-induced compressive stress P2 from the front end portion act on the portions where the flanges 13 are formed which constitute nodes. Due to these stresses, the flange regions undergo compressive stress in the axial direction of the ultrasonic horn 10. Due to this compressive stress, at the flange regions of the ultrasonic horn 10, longitudinal strain is generated in the axial direction, and, together therewith, lateral strain is generated in a direction at right angles to the axis, by the measure of Poisson's ratio. As a result, the ultrasonic horns 10 and 60, in the flange regions, exhibit compressive deformation in the axial direction, and, together therewith, expansive deformation occurs in directions perpendicular to the axis, so that the radius r2 from the centerline of the flange regions is displaced by ε1 in a direction perpendicular to the axis by the expansive deformation, and the dimension thereof becomes r2+ε1. As a consequence thereof, both the front end positions of the flanges 13 and the positions of mounting holes 15 are displaced by substantially ε1 in directions perpendicular to the axis. Because this deformation is caused by the resonance of the ultrasonic horn, displacement appears in the form of vibration. Furthermore, since the flanges 13 are secured to a bonding apparatus, due to such displacement, stress would occur in the flanges 13 and the mounting holes 15.
In order to relieve such stress at the flange regions as described above, several structures are employed. In one structure, as shown in FIG. 24(a), the flange material thickness between the mounting hole 15 and the ultrasonic horn 10 is made thin (as disclosed in U.S. Pat. No. 5,595,328, for example). In another structure, cylindrical portions are provided in the flanges 13, and the ultrasonic horn 10 is supported by those portions (as described in Japanese Patent Application Laid-Open Disclosure (2001) No. 2001-24025. In a still another structure, U-shaped attaching portions 70 are provided outside the flanges 13, and the ultrasonic horn is supported by these portions (see Japanese Patent Application Laid-Open Disclosure (2001) No. 2001-38291). A structure is also employed wherein, as shown in FIG. 26 and FIG. 27, slots 21 are provided between the mounting holes 15 of the flanges 13 and the ultrasonic horn 10 (as shown in U.S. Pat. No. 5,595,328).
All of the above structures seek to relax the stresses that are in the horn securing portions (flanges), providing portions of lowered strength between the ultrasonic horn 10 and the portions for securing to a bonding apparatus, making provision so as to absorb distortion by the deformation of these portions.
There is also an ultrasonic horn in which, as shown in FIG. 28, a slot 21S is made in the flange region of the ultrasonic horn 10 (as disclosed in U.S. Pat. No. 5,595,328). With this structure, the ultrasonic horn 10 deforms on the slot 21S side due to expansion caused by compression, and stress is thus reduced; however, because the ultrasonic horn is secured to flanges along the circumference of the ultrasonic horn, for reasons of flange attachment strength, vibration in the flanges of the ultrasonic horn caused by expansion resulting from the compression described above cannot be suppressed.
The expansive deformation caused by compressive stress resulting from the ultrasonic waves described above is very small; however, when it is generated in the flange regions that constitute horn securing points where the ultrasonic horn is secured to, for instance, a wire bonding apparatus, there are adverse effects on ultrasonic resonance, such as the impedance becoming large and frequencies being generated which are shifted away from the ideal frequency, and bonding quality is caused to deteriorate, which has been a problem. The occurrence of minute vibrations, moreover, causes losses in ultrasonic horn vibration energy which are called “leaks”; as a result, capillary and/or tool vibration becomes insufficient, causing bonding quality to deteriorate, which has also been a problem.
In the meantime, when attempts are made to make wire bonding apparatuses or flip-chip bonding apparatuses operate faster, it becomes necessary that the ultrasonic horn be moved up and down at high speed. Such high speed up-and-down motion then exerts greater force on the supporting portions than conventionally. However, in the related art described above, portions of low strength are provided in the attaching portions to absorb the stresses generated in the securing portions; as a result, the support strength cannot stand up against the large forces occurring due to the faster speeds, and the ultrasonic horn vibrates in the up-and-down direction in conjunction with that up-and-down motion. In a wire bonding apparatus, in particular, when such vibration in the up-and-down direction occurs, excessive force acts on the ball during bonding, crushed ball shape defects occur, and the smaller ball diameters resulting when semiconductor devices become more fine in pitch can no longer be coped with, and that has been a problem. When efforts are made, conversely, to secure adequate strength in the attaching portions, then another problem occurs that vibration in the flange regions is suppressed.